It is known to produce thermoplastic resin in sheet form by compounding such a resin, e.g., polyethylene, with a thermally-decomposing blowing agent such as azodicarbonamide, heat-plastifying the composition under conditions not causing decomposition of the blowing agent, fabricating therefrom a noncellular sheet, cross-linking the sheet if necessary to impart a certain degree of form retentiveness during heating, e.g., by which high energy electron radiation, and heating the sheet to effect decomposition of the blowing agent and formation of gas-filled predominantly closed cells in a resulting foamed resin sheet product. However, it is difficult to heat the sheet uniformly throughout to obtain uniform foaming, particularly in a continuous manner. Moreover, while the resin is in a heat-softened condition, the sheet will sag and distort under its own weight and must be supported in some way even though it is sticky and expanding in all directions.
In. U.S. Pat. No. 3,562,367 to Shinohara et al., there is described a float process of continuously foaming foamable cross-linked polyolefinic resin sheet. In that process, such a sheet is heated in part and supported by floating it on the surface of a heavier liquid heat transfer body, the sheet being concurrently heated from above with heat rays. A suitable such heat transfer liquid is a molten mixture of low melting salts, such as a mixture comprising potassium nitrate and sodium nitrite, which melts at a temperature below the foaming temperature of the foamable sheet and is used at a temperature that softens the sheet and decomposes the blowing agent. In the foaming stage, the sheet is supported upon the flat upper surface of the heat transfer liquid and is free to expand in all directions. However, while heating the foamable sheet to effect uniform foaming thereof, the heat transferred from the liquid to the bottom surface of the sheet must be exactly matched by the heat transferred to the upper surface of the sheet from the irradiating heat source above the sheet, such as infrared lamps or like devices. Attaining such matching is particularly difficult in an operation where the foamable sheet is continuously moved across the surface of a liquid heat transfer bath, foamed, and taken off and away as foamed sheet, wherein the temperature of the sheet changes during such transit and the temperature profiles of the sheet may also change during the operation.
In addition, because the process disclosed in U.S. Pat. No. 3,562,367 employs one heat transfer medium below the foaming sheet and a different medium above the sheet, the problem of temperature matching is further complicated and uniform foaming is harder to achieve. The liquid below the foaming sheet provides better heat transfer than the gas above the sheet, causing the lower surface of the sheet to expand at a faster rate than the upper surface. Thus, curling of the edges of the foaming sheet can occur.
Moreover, sheets of some resins, such as linear polyethylene, which are susceptible to degradation and other deleterious effects of exposure to air at high temperatures, cannot consistently be foamed by the liquid bath surface float method as described in U.S. Pat. No. 3,562,367 without elaborate and inconvenient measures to maintain an appropriately inert atmosphere over the liquid bath.
Accordingly, it would be desirable to provide improved method and means to effect foaming of a heat foamable material especially in the form of sheet comprising thermoplastic resin and foaming agent, such as a thermally-decomposable blowing agent, which method and means would supply the necessary heat uniformly and simultaneously to all sides of the foamable and foaming material while protecting it from exposure to deleterious atmosphere, and would allow the foaming material to expand freely in all directions without distortion while carrying out the operation in a continuous manner.